'draftproof' is a compound formed from 'draft' + 'proof'. 'Draft' (sense of a current of air) comes from the older word 'draught' (Old English 'dræft'/'dræġ', ultimately related to verbs meaning 'to draw' or 'to pull'), and 'proof' (as a combining element meaning 'resistant to') comes via Old French from Latin 'probare' meaning 'to test'.

'draught' (Old English) developed into the noun for a current of air; the American form 'draft' replaced 'draught' in U.S. usage. The suffix '-proof' was used in compounds like 'waterproof' and by analogy 'draft-proof'/'draftproof' arose in modern English.

Initially the elements referred separately to a 'current of air' and to 'being tested' (or 'resistant'); together as a compound they took the specific sense 'resistant to drafts' and have retained that practical meaning.