joli madanm :) "Fouti" is translated as "not able to" based on what you wrote. So I'm reading this sentence (Jan li jenn nan li pa fouti papa l) as (He's not able to be his father) or (He's not able to be a father)? and what is JAN here?

It sounds to me like "He could not be his/her father"

Here are some examples:

1.  Jan mesye sa a jenn nan, li pa fouti papa w.
      The way this man is young, he could not be your father (Lit.)
     This man is so young, he could not be your father.

2. Jan yo t'ap mache men nan men an, yo p'at fouti frè ak sè.
    For the way they were walking hand in hands, they could not be brother and sister.

3. Jan l t'ap desann byè yèreswa a, li pa t'ap  fouti al travay nan demen.
    For the way he was gobbling down beers last night, he could not have gone to work the next day.
    or does the English says: "He could not have been able to go to work..." (I'm not sure :)

4.  Jan w t'al dòmi ta, m konnen ou pa t'ap fouti leve bonè.
     For how you went to bed so late, I know you could not have woken up early.
      You went to bed so late, I know you could not have woken up early.

5. Jan yo te damou an, nou pa't fouti kwè yo te kite vrèman.
    For the way they were in love, we could not believe that they had really broken up.
    They were so much in love, we could not believe that they had really broken up.

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