Sea, cf. foclóir an Duinnínigh:
http://glg.csisdmz.ul.ie/flash/hi/0067.jpg
Bac, -aic, m., -aice, f., orig. neuter ; a quirk, an angular space, hollow or object ; a river turn ; b. n-aÄ‹aiḋ, a paddock ; mioḋ-ḃac, mid-hollow (of the back) ; fioḋ-ḃac, a bow ; a crozier, a mattock, a billhook, a prop, a pin, a crook, a peg, a thole-pin ; a joint, a hook ; a shackle, a hindrance, a stop ; a fire-hob, a fire-prop, corner of hob ; act of supporting, holding back, hindering ; tá ár mb. ar Ḋia, God is our support (Cm.) ; tá de ḃac air, he is being held back from coming, etc. ; fá ḃac an ḟraoiÄ¡, buried under the heather. See baic.