Know on which side your bread is buttered

This proverbial saying is first found in John Heywood’s 1546 glossary A Dialogue conteinyng the nomber in effect of all the Prouerbes in the Englishe tongue:

Thou farest to well (quoth he) but thou art so wood [crazy],
Thou knowst not who doth ye harm, who doth ye good
Yes yes (quoth she) for all those wyse words vttred,
I knowe on whiche syde my breade is buttred.

Heywood spent most of his working life at the court of the Tudor monarchs of England. The factionalism between the Protestant and Catholic supports was intense. It was certainly a time to be clear where one’s interests lay and which horse to back – getting it wrong could be fatal. No better time to coin a phrase like ‘I know which side my bread is buttered’.

See also: the List of Proverbs.