The only free and comprehensive online etymological dictionary of the Spanish language
cabo m. (Noun) "cape"
10th cent. Vulgar Latin caput, recorded in the Appendix Probi of the 3rd or 4th cent., which may have meant "the end" as well as "the beginning" (cf. Italian far capo "to commence"). From Latin caput "head." From Proto-Italic *kaput 'id.' From an unknown pre-Indo-European substrate language *kap- "cup."
Asturian cabu, Portuguese cabo, Galician cabo, Catalan cap, French chef, Italian capo, Aromanian cap, Romanian cap, Sardinian kabu
Old Irish cúäch "cup," Middle Welsh cawg 'id.'
Gothic haubiþ, Old Norse hǫfuþ, Old High German haubit, Old Frisian hāved, Old English hāved (English head)

According to Beekes (1996), there are five reasons why the word is borrowed from another language. First, cognates in other languages point to *a in Proto-Indo-European - a vowel which did not exist. Second, if one reconstructs *kh2p- we must account for a missing full-grade **keh2p-. Third, an incongruous a/au variation (Old English hafut versus Gothic haubiþ) cannot be satisfactorily explained by contamination and there is no reason to assume an ablaut that ended in a stop. Fourth, the suffix -ut in Latin caput is utterly alien to Italic tongues except as the perfect participle, which makes little sense. Admittedly it could be a borrowing from Celtic, but that does nothing to explain the suffix *-it in Germanic, as evidenced in Gothic haubiþ. The most parsimonious explanation is that suffixes *-ut and *-it are remnants from a non-Indo-European language. Fifth and finally, k-initial plain velars words are rare in Proto-Indo-European. Their presence should raise suspicion. On the other hand, Kroonen (2014) believes the words are of native Indo-European stock, as he assumed variation in *a/au was due to supplanting the old proterodynamic paradigm with -au- (borrowed from the oblique).

"Given this situation [above] - which has always been clear - it seems certain the word is a loan. I find it surprising that I nowhere have seen even the suggestion that the word could be non-IE." ~ R. Beekes, "Ancient European Loanwords" (1996)