Green’s Dictionary of Slang

(US campus) to sleep, to lie in bed (longer than considered acceptable).

[1822		Durham Co. Advertiser 5 Jan. 4/3: He lay not all night slugging in a cabin under his mantle, but used commonly to keep others waking].
1956		C.S. Lewis Till We Have Faces 267: ‘Does he mean to slug abed for the rest of his life?’.
1986		Eble Campus Sl. Oct. 5: slug – lie in bed, sleep.