In addition to the etymology given by Merriment, "Latchman" and "Lachman" are also Jewish names (both first names as well as surnames), and are supposedly a literal translation of "Isaac" (meaning, "he laughs"). Although, as noted, the actual meaning of the British surname "Latchman" has nothing to do with laughter.
Although I couldn't find you any online sources stating that Latchman is a secular name used by Jews for "Isaac", I did find you a couple of sources for the surname:
http://www.gaminggeeks.org/Resources/KateMonk/Middle-East/Jewish/Surnames.htm
http://www.shtetlinks.jewishgen.org/Kolomea/nameorigin.htm
-- Nanaea