"a sailor, one whose traffics and voyages are ships on the sea," Middle English seman, from Old English sæmanna (plural); see sea + man (n.). Similar formation in Dutch zeeman, German Seemann, Old Norse sjomaðr. In later times technically restricted to men below the rank of officer. Related: Seamanly; seamanlike.
"having sufficient power or means," early 14c., from Old French (h)able "capable; fitting, suitable; agile, nimble" (14c.), from Latin habilem, habilis "easily handled, apt," verbal adjective from habere "to hold" (from PIE root *ghabh- "to give or receive").
"Easy to be held," hence "fit for a purpose." The silent h- was dropped in English and resisted academic attempts to restore it 16c.-17c. (see H), but some derivatives (such as habiliment, habilitate) acquired it via French. Able seaman, one able to do any sort of work required on a ship, may be the origin of this:
Able-whackets - A popular sea-game with cards, in which the loser is beaten over the palms of the hands with a handkerchief tightly twisted like a rope. Very popular with horny-fisted sailors. [Smyth, "Sailor's Word-Book," 1867]
"seaman, sailor, one who directs or assists in navigating a ship," mid-13c., from Anglo-French mariner, Old French marinier "seaman, sailor" (12c.), from Medieval Latin marinarius "sailor," "of the sea, maritime," from Latin marinus "of the sea," from mare "sea, the sea, seawater," from PIE root *mori- "body of water." Earlier and long more common than sailor. A sailor also could be a brimgeist in Old English.
"soft-bodied invertebrate animal, usually with an external shell," 1783, mollusque (modern spelling from 1839), from French mollusque, from Modern Latin Mollusca (see Mollusca), the phylum name. Related: Molluscuous; molluscan.