This is an abbreviation on internet chat sites for Age/Sex/Location. If you're a creepy paedophile and you want some juicy youngster to provide you with some basic contact details (to be more thoroughly investigated later) you simply pretend that you're a nice, wholesome 14-year-old boy, establish a bit of a rapport with the target, then type:
The juicy youngster will then gullibly respond with something like:
12/f/Houston, wanna cyber?
At which point you're as good as in her lovely, pre-pubescent knickers. Job's a good 'un.
NB: On dedicated sex sites, a/s/l may alternatively stand for Anal/Standing/Lying down. Or so I'm reliably informed.
ASL is also used to refer to "American Sign Language", a complex series of manual gestures used by the sensorily disabled to transmit absolute gibberish while making those around them uneasy because they think they're being talked about. One of the crueler idiosyncrasies of the deaf. *sniff*