1) Farfetched 2) Incredible 3) Lame 4) Unbelievable 5) Unlikely 6) Unthinkable 1) Full of holes 2) Improbable 3) Incredible 4) Not plausible 5) Unbelievable 6) Unconvincing 7) Unlikely 
• (a.) Not plausible; not wearing the appearance of truth or credibility, and not likely to be believed.
Im·plau'si·ble adjective [ Prefix im- not + plausible : confer French implausible .] Not plausible; not wearing the appearance of truth or credibility, and not likely to be believed. ' Implausible harangues.' Swift. -- Im*plau'si*ble*ness , 
[adj] - having a quality that provokes disbelief
implausible 1. Having a quality that provokes disbelief: 'He gave the teacher an implausible excuse for being tardy.' 2. Highly imaginative but unlikely: 'The scientist had an implausible explanation for the natural incident.' 3. Lacking any appearance of truth or credibility.
adjective having a quality that provokes disbelief; `gave the teacher an implausible excuse`
not plausible; not having the appearance of truth or credibility: an implausible alibi.