You haven't heard of brownskirts coz I personally made it up. I've only used it here and on reddit GC before the ban.

So when boys start jerking off as young teens, the AGP thing can be part of it? Well, yeah I have been reading on this forum about boys stealing their sister's clothes. I wonder how many fear they're gay and that's why they try to keep it under wraps?

Yes, that's always how AGP starts for boys - and often before they're teens coz for both sexes puberty usually starts well before the teens. Male puberty is considered early or precocious only if it starts before a boy turns 9. Lots of kids of both sexes masturbate before puberty too. Boy children commonly get spontaneous erections from an early age (for some, even in the womb), and play with their penises throughout childhood (again, scans have shown males doing it in the womb). For boys

Masturbation becomes goal-driven around age 10. Boys in particular are trying to get to the point of orgasm, and at around 11 or 12, they may start seeking pornographic material. "That's when the Internet becomes a big deal, and parents need to be careful about what kids can access online,"

https://www.webmd.com/parenting/features/caught-your-kid-masturbating

Sexologists J. Michael Bailey and Ray Blanchard give general description of how AGP starts here:

Autogynephilia is a male’s sexual arousal by the fantasy of being a woman. That is, autogynephilic males are turned on by thinking about themselves as women, or behaving like women. The typical heterosexual adolescent boy has sexual fantasies about attractive girls or women. The autogynephilic adolescent boy’s may also have such fantasies, but in addition he fantasizes that he is an attractive, sexy woman. The most common behavior associated with autogynephilia during adolescence is fetishistic cross dressing. In this behavior, the adolescent male wears female clothing (typically, lingerie) in private, looks at himself in the mirror, and masturbates.

https://4thwavenow.com/2017/12/07/gender-dysphoria-is-not-one-thing/

I wonder how many fear they're gay and that's why they try to keep it under wraps?

Since the sexual fantasies and attractions of most males who develop AGP are of, and towards, girls and women, I think most of these boys know they are not gay. But since immature boys often fear anything that could possibly seem "gay" or bi, or anything other than 100% "vanilla" heterosexual, that might be a factor.

However, it should be pointed out that Bailey and Blanchard's description of how AGP starts off given above is too simplistic. My impression is, AGP and its associated masturbatory habits tend to develop in progressive stages, so there's often quite a lag between being aroused by the sight of mum's, sister's, aunt's or grandma's panties, bra, nightgown and pantyhose, and a boy rubbing his penis on those items to get off, to a boy actually donning those items to get off. And there's a lag between a boy doing that last step and progressing to donning female attire and situating himself in front of a mirror so he can watch himself beating off dressed that way. Unfortunately, Bailey and Blanchard's description makes it sound as if adolescent boys who develop AGP engage in full-fledged cross-dressing and jerking off in front mirrors from the get-go. (Although I'm not a sexologist myself, I have spoken to a lot of men about their private thoughts and habits in adolescence and adulthood, I grew up in close company of and surrounded by boys, and I've raised sons and talked to lots of other parents of sons - so I'm not just talking through my hat here.)

My sense is, boys who develop AGP start off with the usual amount of shame that most kids have around masturbating, but this changes when they reach the point clearly imagining themselves as female in their sex fantasies and putting on items of female clothing to masturbate in. At that point, their sense of shame most likely gets compounded and becomes much, much larger and deeper. But my sense is this isn't so much because they fear they might be gay or bi per se, but because they feel and fear their fantasies, behavior and the intense sexual pleasure that they feel as a result mean they are abnormal, weird and deviant.

Of course, the fact that most boys see being a girl or woman itself as inherently shameful and degraded plays a large role as well. My hunch is that boys who develop AGP tend to feel horrified and ashamed when they find themselves fantasizing about being female and wanting to don female intimate attire in the first place - but when they discover that such fantasies and dressing up behaviors sexually arouse them like nothing else in the world arouses them, they become even more horrified and ashamed. And that, IMO, is where a lot of the urge to keep it hidden comes from.