That doesn't define anything : "rainy means it's rainy" may be as clear
Using several dictionnaries (free dictionnary, Merriam, MacMillan, Oxford)
UNLIKELY
1. Not likely; improbable.
2. Not promising; likely to fail.
1 : not likely : improbable <an unlikely outcome>
2: likely to fail : unpromising
1 : not likely to happen
2 : not typical
Not likely to happen, be done, or be true; improbable:
IMPLAUSIBLE
1. not plausible; provoking disbelief; unlikely
1 : not plausible : provoking disbelief
1 : difficult to accept as true
(Of an argument or statement) not seeming reasonable or probable; failing to convince:
You're confusing two distinct propositions.
It's implausible/unlikely for this precise person to drown in his own bathtub.
It's implausible/unlikely for one person to drown in his own bathtub.