Dictionary:

undependable

  (ŭn'dĭ-pĕn'də-bəl)

Not easily relied or depended on: an undependable worker; an undependable lamp socket.

adjective

1. Not to be depended on: unreliable, untrustworthy. See trust/distrust.
2. So weak or defective as to be liable to fail: trick, unreliable. See strong/weak.

Definition: irresponsible
Antonyms: dependable, reliable, responsible, trustworthy

The adjective has 3 meanings:

Meaning #1: not worthy of reliance or trust
  Synonym: unreliable
  Antonym: dependable (meaning #1)

Meaning #2: not to be trusted
  Synonym: unreliable

Meaning #3: liable to be erroneous or misleading
  Synonym: unreliable