Unreliable, untrustworthy (3-2-5)
I believe the answer is:
fly-by-night
I believe this is a double definition.
'unreliable' is the first definition.
(I've seen this before)
'untrustworthy' is the second definition.
(similar in meaning)
(Other definitions for fly-by-night that I've seen before include "Untrustworthy person, likely to abscond" , "Unreliable - person who departs secretly to avoid debts" , "Unreliable, shady" , "Transitory" , "Unreliable (in business)" .)