weak kneed

indecisive; not thorough

Adjective
1. lacking will power or resolution; "the role of the dissenter is not for the weak-kneed"
(similar) irresolute

(a.)
Having weak knees; hence, easily yielding; wanting resolution.
  

not strong in character, lacking will power Marvin yielded to temptation. He appears to be weak kneed.

Synonyms and related words:
abulic, afraid, chicken, chickenhearted, coward, cowardly, cowed, daunted, dismayed, double-minded, faint, fainthearted, faltering, fearful, feeble, feebleminded, frail, funking, funky, henhearted, infirm, intimidated, invertebrate, irresolute, lily-livered, milk-livered, milksoppish, milksoppy, mousy, overtimid, overtimorous, panic-prone, panicky, pigeonhearted, pliable, rabbity, sissified, sissy, soft, spineless, timid, timorous, uncertain, unmanly, unmanned, wavering, weak, weak-minded, weak-willed, weakhearted, white-livered, wobbly, yellow