Old English mara "greater, more, stronger, mightier," used as a comparative of micel "great" (see mickle), from Proto-Germanic *maizon- (cf. Old Saxon mera, Old Norse meiri, Old Frisian mara, Middle Dutch mere, Old High German mero, German mehr), from PIE *meis- (cf. Avestan mazja "greater," Old Irish mor "great," Welsh mawr "great," Greek -moros "great," Oscan mais "more"), from root *me- "big." Sometimes used as an adverb in Old English ("in addition"), but Old English generally used related ma "more" as adverb and noun. This became Middle English mo, but more in this sense began to predominate in later Middle English.
More or less "in a greater or lesser degree" is from early 13c.; appended to a statement to indicate approximation, from 1580s.