2 Answers

lad

noun
1.
informal
a boy or young man (often as a form of address).
"come in, lad, and shut the door"
synonyms: boy, schoolboy, youth, youngster, juvenile, stripling, young fellow, junior, whippersnapper; More
BRIT.
a group of men sharing recreational, working, or other interests.
"she wouldn't let him go out with the lads any more"
BRIT.
a boisterously macho or high-spirited young man.
"Tony was a bit of a lad —always had an eye for the women"
synonyms: man, young man; More
2.
BRIT.
a stable worker (regardless of age or sex).
"it's great for the lads that the horse has won the National"

Or it could mean any of these listings >>>http://acronyms.thefreedictionary.com/LAD

Also the Language Acquisition Device (LAD) is a postulated "organ" of the brain that is supposed to function as a congenital device for learning symbolic language
search it and see wikipedia this is where it comes from. Are you asking about this or language acquisition deficiency ?