'nonalphabetic' is formed in modern English by adding the negative prefix 'non-' to 'alphabetic'. 'Non-' comes from Old English/Latin usage as a negating prefix meaning 'not', and 'alphabetic' ultimately derives from Greek 'alphabetos' (the word for the sequence of letters beginning with 'alpha' and 'beta').

'alphabet' comes from Greek 'alphabetos' → Latin 'alphabetum' → Old French/Medieval Latin forms → Middle English 'alphabet'; 'alphabetic' developed from 'alphabet' + '-ic' in Late Latin/English. The compound 'nonalphabetic' is a modern English formation created by prefixing 'non-' to 'alphabetic' to express the negative.

Originally 'alphabetic' meant 'of or relating to the alphabet'; with the addition of 'non-' the word came to mean 'not of or relating to the alphabet' — i.e., composed of non-letter characters.