Deriving etymology is probably the most fun a person can have without having to take their clothes off. Take, for instance, the etymology of the name ‘James’ and ‘Jacob’ --- both ultimately come from the Hebrew ‘Ya’aqov’ (יעקב), meaning ‘holds the heel’. The Greeks borrowed his name as ‘Iakob’ (Ιακώβ) for the Septuagint, and the Romans borrowed it as ‘Jacobus’ (or IACOBVS, if you prefer). Remember your Old Testament? We know this guy as Jacob, who fought with an angel of God and after having his hip dislocated by a divine body-slam, was renamed ‘Isra’el’ (ישראל), which means ‘wrestles God’.
Good guy to have on your side in a bar fight.
Now, the Romans and Greeks were pretty sloppy with those barbaric foreign names and in a lot of cases, had more than one form. Alongside ‘Jacobus’, there was another form, ‘Jacomus’. ‘Jacobus’ gave us ‘Jacob’ and the Italian 'Giacomo', whil ‘Jacomus’ gave us ‘James’ and the Spanish 'Jaime'.
Anyway, so we have these two forms that give us ‘Jacob’ and ‘James’, both from the same root. But it doesn’t stop there. Ya’aqov lent his name to Santiago (at least that last element, ‘Iago’) and further on to San Diago and San Diego, both of which were derived from ‘Santiago’, with the final consonant of the first element becoming the initial consonant of the second in a process called ‘sandhi’. Sort of the same thing happened in English with the word ‘adder’ (a type of snake), which was originally called a ‘nadder’ (akin to the German ‘Natter’). hapax suggests 'napron' as a better example: 'a napron' eventually resulted in 'an apron'.
However, some say that ‘Diego/Diago’ is from the Latin 'Didacus', ultimately borrowed from the Greek ‘didakhos’ (διδάχος), which means ‘learned’. This is plausible and fits pretty well with the sound laws that governed the gradual evolution from Latin to Spanish, but since these three forms (Iago, Diego and Diago) were attested in writing in roughly the same time period, the former hypothesis strikes me as a little more likely: Ya’aqov gave us ‘Jacob’, ‘James’, ‘Diago’, ‘Diego’ and ‘Iago’.
Nice work for a guy who had the
cojones to try and put the smack down on an angel of
Yahweh (unbeknownst though it was at first).
Thanks to hapax for supplying a better example of sandhi than 'adder', and yes, DonJaime, I deliberately left 'Jaime' out of my examples to spite you.