AHD says
Alteration (influenced by BUCKLE) of parbunkel.] . I couldn't find any etymology listed for turnbuckle, but given the def. I have to say I think it's pretty obvious.
I decided to try looking up par, when it's used as a prefix, since buckle has a clear meaning here. Two dictionaries directed me to LU para-. This is from infoplease:
para-
1. a prefix appearing in loanwords from Greek, most often attached to verbs and verbal derivatives, with the meanings “at or to one side of, beside, side by side” (parabola; paragraph; parallel; paralysis), “beyond, past, by” (paradox; paragogue); by extension from these senses, this prefix came to designate objects or activities auxiliary to or derivative of that denoted by the base word (parody; paronomasia), and hence abnormal or defective (paranoia), a sense now common in modern scientific coinages (parageusia; paralexia). As an English prefix, para- 1 may have any of these senses; it is also productive in the naming of occupational roles considered ancillary or subsidiary to roles requiring more training, or of a higher status, on such models as paramedical and paraprofessional: paralegal; paralibrarian; parapolice.
2. Chem.a combining form designating the para (1, 4) position in the benzene ring. Abbr.: p-. Cf. meta- (def. 2c), ortho- (def. 2b). See diag. under ortho-. Also, esp. before a vowel,par-. http://www.infoplease.com/ipd/A0577416.html
