Meaning

noun a line drawn underneath (especially under written matter)

verb give extra weight to (a communication)

verb draw a line or lines underneath to call attention to

originEtymology tree Proto-Indo-European *h₁én Proto-Indo-European *h₁entér Proto-Indo-European *(H)n̥dʰí Proto-Indo-European *-ér Proto-Indo-European *(H)n̥dʰér Proto-Germanic *under Proto-West Germanic *undar Old English under- Middle English under- English under- English line English underline From under- + line.

Etymology from Wiktionary, CC BY-SA 3.0