unwohl
unwell
To feel physically not well, slightly ill, or nauseous.
Nach dem fettigen Essen war mir den ganzen Abend unwohl.
After the greasy food, I felt unwell all evening.

unwell / uncomfortable
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unwell
To feel physically not well, slightly ill, or nauseous.
Nach dem fettigen Essen war mir den ganzen Abend unwohl.
After the greasy food, I felt unwell all evening.

The word is the direct opposite of 'wohl' (well). The prefix 'un-' negates the adjective 'wohl' (good, healthy), so 'unwohl' (unwell/uncomfortable) literally means 'not good' or 'not healthy'.
Er fühlte sich nach der langen Reise sichtlich unwohl.
He visibly felt unwell after the long journey.
'Unwohl' (unwell/uncomfortable) is almost always used in two fixed constructions. Either with the verb 'fühlen' (to feel): 'Ich fühle mich unwohl' (I feel unwell/uncomfortable), or with 'sein' (to be), where the person is in the dative: 'Mir ist unwohl' (I feel unwell).
Ich fühle mich bei diesem Thema unwohl.
I feel uncomfortable with this topic.
The context tells you whether physical or psychological discomfort is meant. If it is about food, illness, or the body, the physical meaning is intended. If it is about social situations, conversations, or feelings, the psychological meaning is intended.
Vom Geruch des Fisches wurde mir unwohl.
The smell of the fish made me feel unwell.
probably / well
'wohl' and 'unwohl' are opposites when it comes to how a person feels. Someone who feels 'wohl' feels good, safe, or comfortable. Someone who feels 'unwohl' feels physically unwell or uneasy and uncomfortable in a situation. The words can be confused because they look very similar and both have to do with feelings. Pay attention to 'un-': it turns the good feeling into the opposite. 'wohl' can also mean 'probably', but that meaning does not fit the contrast with 'unwohl'.

Nach dem langen Gespräch ist der Verkäuferin etwas unwohl.
After the long conversation, the saleswoman feels somewhat uneasy.
In der Pause fühlt sich das neue Teammitglied schon viel wohler.
During the break, the new team member already feels much more comfortable.