mid-13c., monen, "mourn (someone); regret, bewail;" c. 1300, "to lament inarticulately, grieve; utter mournfully;" probably from Old English *mānan, a variant of mænan "to lament" (see moan (n.)). From late 14c. as "complain, tell one's troubles." From 1724 as "to make a low sound expressive of physical or mental suffering." Related: Moaned; moaning.
c. 1200, mon, "lamentation, mourning, weeping; complaining, the expressing of complaints; a complaint; lover's complaint; accusation, charge," perhaps from an unrecorded Old English *mān "complaint," from mānan, a variant of mænan "complain, moan," also "tell, intend, signify" (see mean (v.1)); but OED discounts this connection. Meaning "long, low inarticulate murmur expressing grief or pain" is by 1670s, "with onomatopoeic suggestion" [OED].
"intend, have in mind;" Middle English mēnen, from Old English mænan "intend (to do something), plan; indicate (a certain object) or convey (a certain sense) when using a word," from Proto-West Germanic *menjojanan (source also of Old Frisian mena "to signify," Old Saxon menian "to intend, signify, make known," Dutch menen, German meinen "think, suppose, be of the opinion"), from PIE *meino- "opinion, intent" (source also of Old Church Slavonic meniti "to think, have an opinion," Old Irish mian "wish, desire," Welsh mwyn "enjoyment"), perhaps from root *men- (1) "to think."
From late 14c. as "have intentions of a specified kind" (as in to mean well). Of a person or thing, "to be of some account, to matter (to)," by 1888. Conversational question you know what I mean? attested by 1834.