Why do we call a nun's costume (or should I say "uniform"?) a habit? Because it's habitual? It couldn't possibly be because she "inhabits" it, could it?
(I know, I could just look it up, but what would be the shared fun in that? )
The sense development, as seen in Latin and the modern languages taken together, is thus: orig. Holding, having, ‘havour’; hence the way in which one holds or has oneself, i.e. the mode or condition in which one is, exists, or exhibits oneself, a) externally; hence demeanour, outward appearance, fashion of body, mode of clothing oneself, dress, habitation; b) in mind, character, or life; hence, mental constitution, character, disposition, way of acting, comporting oneself, or dealing with things, habitual or customary way (of acting, etc.), personal custom, accustomedness. This development was largely completed in ancient Latin, and had received some extension in OF., before the word became English; in our language, senses were taken, from time to time, from Fr. or L., without reference to their original order of development; hence the chronological order in Eng. is in no way parallel to the original; and the arrangement below is only partly chronological. In mod.F. the word is narrowed down to our branch I, other senses being supplied by habitude; thus Eng. ‘habit’ is co-extensive with the two French words, and its chief sense corresponds not to F. habit but to F. habitude.
Etymologically, habit is what is had by you or others.
Still without any research, I'd guess that habit is descended from the Latin habeo, "I hold, or own". Since so few people at the time had much other than the clothes they stood up in, the meaning transferred to "clothes". Later, as the peasantry got more affluent, it fell out of use, apart from by monks and nuns, who had renounced all worldly goods, and so had nothing bar their clothes that they could call their own.
The (rather impenetrable) definition from the OED explains how the other main meaning came about, I'm sure.
Build a man a fire and he's warm for a day. Set a man on fire and he's warm for the rest of his life.
A wise man once said that there are two kinds of knowledge: we either know something of itself, or we know where to look it up. One does not have to agree with what others have said about something, but I usually take that as a jumping off spot in the investigation of that same something. That's just how I work, and I'm sorry if that makes me a spoilsport.
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