"exchange of heavy blows," by 1910, originally in reference to baseball, from slug (n.3) + -fest. Slugging-match "unskillful fistfight" is by 1878.
Entries linking to slugfest
slug n.3
"a hard blow," 1830, dialectal, of uncertain origin; perhaps related to slaughter or perhaps a secondary form of slay.
-fest
word-forming element in colloquial compounds (hen-fest, gabfest, etc.), from 1889, American English, borrowed from German Fest "festival," abstracted from Volksfest, etc., from Middle High German vëst, from Latin festum "festival or holiday," neuter of festus "of a feast" (see feast (n.)).