Only ever heard it over here in Ireland - mainly by Dubs.
Luke would be the person to ask - there's plenty of them in Dublin!
This is another definition from Bernard Share's 'Slanguage' (highly recommended).

Jim Carroll
Latchico [n., cf. Sc. latch, indolent, idle person, poss. <OF laschier (vb.), relax; but many other etyms. suggested; 'Dockland slang for a waster or a rogue' (Vincent Caprani, Rowdy Rhymes & 'Re - im-itations',1982); used on Brit, building sites by West of Ireland workers, poss. <unidentified Ir. (DOM)]. As thus; halfwit. 1970 Christy Brown, Down All the Days: '"They trust me, bejasus [q.v.]...They know I'm no fucking latchico or general foreman's lick-arsc [see lick].'" 1989 Robert E. Tangney, Other Days Around Me: '"What did you let him in for?" roared the angry widow. "He's the biggest latchico in town.'"

Jim: Your problem was a couple of “less than” signs which fooled your browser (and everyone's browser) into thinking you were posting an HTML command. If you need to use a “less than” sign that is NOT the beginning of an HTML command, you need to replace “<” with “&lt;”. --One of the JoeClones