"Чаплин" по-английски капеллан, священник. Русская же фамилия происходит от диалектного "чапля" - цапля.
http://dic.academic.ru/dic.nsf/lastnames/14127

chaplain (n.)
mid-14c., from Old French chapelein "clergyman" (Modern French chapelain), from Medieval Latin cappellanus "clergyman," originally "custodian of St. Martin's cloak" (see chapel). Replaced Old English capellane, from the same Medieval Latin source.