To surreptitiously scratch your testicles through trouser pockets. Not to be confused with surreptitious wanking.
"What's up Fred, you lookin' for change?"
" Nah mate, just 'avin a good fossle!"
To surreptiosly scratch your testicles through your pockets when that annoying, spontaneous, itch thing, happens.
"Wow! I know I look good but I didn't think that I was hot enough to make men spontaneously masturbate!"
"Dont kid yourself you ugly cow, I'm just fossling!"