"Once a Dodger, Always a Dodger" (1934)

[Benjamin Zimmer]

Hmmm, don't know why I didn't think to check the OED first...

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_once_, adv., conj., a., and n.

16. Proverb. _once a --, always a --_ and variants, indicating that a
particular role cannot be or is unlikely to be relinquished.

[1566 W. P. tr. C. S. Curio Pasquine in Traunce f. 107v, The olde rule: he
that is once a false knaue, it is maruell if euer he be honest man after.]
1613 H. PARROT Laquei Ridiculosi II. cxxi sig. N2v, Well you may change
your name, But once a Whoore, you should be still the same. 1622 J. MABBE
tr. M. Aleman Guzman d'Alf. I. I. II. i. 7 Once a knaue, and euer a
knaue:..For he that hath once beene naught, is presumed to bee so still.
1696 Cornish Comedy IV. i. 30 ‘I'll do so no more.’ ‘Not till next time;
once a Villain, and always so.’ 1705 P. A. MOTTEUX Amorous Miser III. 58
Once a Captain and always a Captain. 1760 W. KENRICK Falstaff's Wedding
IV. i. 52 As to the matter of knighthood; once a knight and always a
knight, you know. [etc.]
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--Ben Zimmer