'Deficit' is a Latin verb — 'it is wanting.' A living verb fossilised into an accounting noun.
The amount by which something, especially a sum of money, falls short of what is required; a deficiency or shortfall.
From Latin 'deficit' (it is wanting, it is lacking, it falls short), the third-person singular present indicative active of 'dēficere' (to fail, to be wanting, to desert, to run short), compounded from 'dē-' (down, away from — here expressing departure, removal, or falling below a threshold) and 'facere' (to do, to make, to perform, to bring about). The PIE root of 'facere' is *dʰeh₁- (to set, to place, to put, to make) — among the most generative roots in Latin, underlying 'fact,' 'factor,' 'faculty,' 'affair,' 'feat,' 'fashion,' 'manufacture,' 'satisfy,' 'benefit,' 'office,' 'perfect,' and dozens more English words. The word 'deficit' was borrowed directly into English as a Latin third-person verb form reused as an English noun — a practice common in early modern double-entry accounting, where Latin
Like 'video' (Latin 'I see'), 'deficit' is a conjugated Latin verb form used as an English noun — 'deficit' literally means 'it is lacking.' Financial Latin was full of such verb forms used as accounting terms: 'debit' (he owes), 'credit' (he believes/entrusts), and 'deficit' (it falls short) were all present-tense Latin verbs that became English nouns.