1
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Step back, retreat. For example, When it was time for a group picture, Mary dropped back.
American Heritage Idioms
2
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Phrase(s): drop back
1. to fall back to an original position. • His arm raised up and then dropped back. • The lid dropped back to its original position as soon as we let go of it. 2. to go slowly and lose one’s position in a march or procession. • He dropped back a bit and evened up the spacing in the line of marchers. • He got tired and dropped back a little.
McGrawhill's American Idioms And Phrasal Verbs