1408, perhaps a causative form of M.E. suken "to suck" (see suck), or a back-formation from suckling (though this word is attested only from c.1440).
suckling
c.1440, "an infant at the breast," from suck + dim. suffix -ling. Cf. M.Du. sogeling, Du. zuigeling, Ger. Säugling. Meaning "act of breast-feeding" is attested from 1799.