Synonyms of guaranteeing

1 Syllable
swear, back
2 Syllables
support, aver, angel, cosign, witness, bankroll, warrant, wager, protect, confirm, juice, affirm, prove, stake, attest, maintain
3 Syllables
endorse, testify, grubstake, certify, evince, assure, secure, reassure, insure, mortgage, guaranty, ensure
4 Syllables
evidence