means A slug is a slow-moving, shell-less land mollusk that leaves a trail of slime, or by extension a sluggish person, a lump of metal, or a single drink swallowed in one gulp.

from From Middle English 'slugge,' meaning a lazy or slow person, likely borrowed from a Scandinavian source — compare Norwegian dialect 'slugg,' a heavy, slow body. The creeping garden creature only took the name later, around the 18th century, because it embodied the same unhurried laziness. The senses for a lump of metal (and the bullet kind) and the boxing 'slug' grew from the idea of a heavy, formless mass or a heavy blow, with 'slug' meaning 'to hit hard' appearing in 19th-century American slang.