The Spanish word 'adonde' (meaning 'where to') is a combination of two elements: 'a' (meaning 'to') and 'donde' (meaning 'where'). The 'a' comes from Latin 'ad' meaning 'to' or 'toward', while 'donde' has an interesting origin combining two Latin words: 'de' ('from') and 'unde' ('whence'). Over time, these Latin elements merged and evolved to create the modern Spanish question word that specifically asks about direction or destination.

The most obvious related Spanish word is 'donde' (meaning 'where'), which is actually part of 'adonde'. While 'donde' simply asks about location, 'adonde' specifically asks about direction or destination. Another related word is the simple preposition 'a' (meaning 'to'), which gives 'adonde' its directional meaning.

While there aren't direct cognates in modern English for 'adonde', we can see a distant relationship in some English words derived from Latin 'ad-' prefix, such as 'address' (which involves direction toward something) and 'advance' (moving toward something). The concept of 'whence' in Old English, though now archaic, is related to the Latin 'unde' that helped form 'donde'.