Latin costa 'rib; side, flank' has been traditionally liked by Indo-Europeanists to Old Church Slavonic kostĭ 'bone'1, although more recently De Vaan considers this etymology to be dubious, among other reasons because of semantic mismatch2.
In fact, the Slavic word is undoubtedly linked to IE *H2ostH1- 'bone', although its initial velar stop can't be accounted for in the mainstream model. However, the amateur linguist Glen Gordon explains this alternation *k- ~ *H2-3 as a consequence of an early borrowing from a femenine variant of Semitic *kˀa(w)ʃ- 'bow'4 (cfr. Phoenician qšt5), with a semantic drift 'bow' > 'rib' > 'bone'.
In most Romance languages except Romanian, the meaning 'rib' of Latin costa was transferred to diminutive forms (e.g. Spanish costilla, French côtelette), while the main word specialized into geographical meanings: '(hill) slope' (e.g. Spanish cuesta) and 'shore' (Catalan and Italian costa), which spread as a Wanderwort: Spanish costa, English coast, Dutch and Swedish kust, German Küste, Danish kyst, etc.
There's in addition Middle High German Gestade 'bank' and Old Irish ces 'flank, rump steak', césán 'flanks', which can't be readly derived from costa although they're semantically and phonetically close. This makes me wonder if all these words could be Semitic loanwords akin to or from Phoenician qsˁt 'edge, limit' (f.)6 < Semitic *qītʃˀ- 'end, to finish' < Afrasian *kˀajatʃˀ- (HSED 1562), with Latin and German having got the femenine variant and Goidelic the masculine one.____________________________________________
1 A. Ernout & A. Meillet (1959): Dictionnaire étymologique de la langue latine, p. 146.
2 M. De Vaan (2008): Etymological Dictionary of Latin and the other Italic Languages, p.140.
3 Regarded by Gamkrelidze-Ivanov as different reflexes of a former *qʰ. T.V. Gamkrelidze & V.V. Ivanov (1995): Indo-European and the Indo-Europeans, pp. 111-113.4 G. Gordon (2008): Possible Proto-Semitic Loanwords in Pre-IE.
5 Mª J. Estanyol (2008): Diccionari abreujat fenici-català, p 113.
6 Mª J. Estanyol, op. cit., p 112.