tinanam0102

“ Positive, not sarcastic.  Right.

to hold and repress the gas whey they sang.    I love it! ”

There's a usage similar to "He's a soft touch."

When a con man or a scam artist is looking for a mark (person to run his scam on), he looks for a gullible person (a sucker), or sometimes someone with a "big heart."  Panhandlers can usually tell by looking at a person whether or not he's likely to give them money.

We might say that a person has a "weakness," or a "soft spot in his heart" for some particular thing or situation.  "He can't resist readheads."  "He's a sucker for readheads."

My mother used to have an expression, "He's a sucker for punishment," meaning someone mistreats him and he keeps coming back for more.  "He doesn't know when he's had enough."

tinanam0102

“ "Go ahead.  I'm a sucker for you". As you continue to fill in the details, I'm less inclined to say he thought badly of you.  It's still a little ambiguous. People are naturally suspicious of car dealers, and not without reason.  But it could be a good-natured reply   -   especially if your relationship has been good up to this point.

It could be used in the sense below:  He likes you and he trusts you, even though he thinks he may be foolish to do so.

"He can't resist readheads."  "He's a sucker for readheads."  (I couldn't find this entry in the dictionary)  A "redhead" is a person with natural red hair.  Whenever I see a redheaded girl, I go all weak in the knees.

he keeps coming back for more.  "He doesn't know when he's had enough.  (What do you mean?   The idea is that a sensible person would understand that when someone continually abuses him, the prudent thing would be to stay away.  But this guy is foolish.  He doesn't know when to quit. ”

Sometimes we are overcome by emotion and seem to lose our resolve.  We lose our ability to act with calm self-control and steady determination.  Sometimes seeing a girl can effect you like that.  Or it may be fear of something.  You may be actually physically trembling, or weak in the knees; or it may be a sort of mental incapacitation.  Maybe you "freeze," unable to act.

I'm only familiar with "weak in the knees."  Perhaps the others are BrE.