You can use the word pantywaist to talk about an ineffective, weak person — especially a man — although it's not very nice and is likely to hurt his feelings.

The noun pantywaist has proven to be more enduring in its figurative meaning — an ineffectual or childish man — than in its original meaning, which was a garment of early 20th century vintage: short pants fastened to a waist band, worn by children. Still, the term is somewhat old fashioned, and calling a timid man a pantywaist these days would not only be unkind, but might also be confusing.

Definitions of pantywaist
  1. noun
    a timid man or boy considered childish or unassertive
    synonyms: Milquetoast, milksop, pansy, sissy
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    type of:
    a person who shows fear or timidity
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