Wiktionary
vb. (context idiomatic English) To divulge a secret, as by accident or mistake.
Usage examples of "let slip".
Instead she took a deep breath and quietly let slip her husband's favorite swearword.
Our young men are very greedy of honor, and do not let slip such opportunities.
And such a numskull is he as fondly to believe that the girls are so simple as to let slip their opportunities, while they wait on the caprice of father or brothers, who six times out of seven delay to marry them for three or four years after they should.
Perhaps it was at first only the manifest reluctance of my old preceptor to discuss with me my paternal ancestry that gave rise to the terror which I ever felt at the mention of my great house, yet as I grew out of childhood, I was able to piece together disconnected fragments of discourse, let slip from the unwilling tongue which had begun to falter in approaching senility, that had a sort of relation to a certain circumstance which I had always deemed strange, but which now became dimly terrible.
The chance for a joke was altogether too fine a one to be let slip by the goodly company of lawyers' clerks.