To pitch in, to go ahead.
1889. A man must dismiss all thoughts of prudence and common-sense when it comes to masquerade dresses, and just sail in and make an unmitigated fool of himself, and have it out with his tailor and his governor afterward.—J. Lillie, ‘Slowtopp’s Confession,’ Harper’s Mag., lxxviii. 561/1 (March). (N.E.D.)
