The term "nucleophilic" describes substances that have excess electron pairs that they want to donate or share.
A proton does not have any excess electron pairs, so it is not nucleophilic.
A carbanion has a pair of excess electrons on a carbon atom, so it is nucleophilic.
An imidazole has excess electrons on its nitrogen atom, so it is nucleophilic.
Therefore, only the proton is not nucleophilic.
