Teaching grandmother to suck eggs is an English language saying meaning that a person is giving advice to someone else about a subject of which they are already familiar (and probably more so than the first person).
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“But what am I about? If my grandmother sucks eggs, was it I who taught her?”
Origins of the phrase
The origins of the phrase are not clear. The OED and others suggest that it comes from a translation in 1707, by J. Stevens, of Francisco de Quevedo (Spanish author):
"You would have me teach my Grandame to suck Eggs"
Notable early uses
“I remember my old schoolmaster, who was a prodigious great scholar, used often to say, Polly Matete cry Town is my daskalon. The English of which, he told us, was, That a child may sometimes teach his grandmother to suck eggs”
“But what am I about? If my grandmother sucks eggs, was it I who taught her?”
Related phrase
The use of the phrase "Suck-egg" for "a silly person" dates back to 1609, in the OED.