(Note: in this posting I’m going to be unrelentingly careful about the way I frame descriptions of linguistic phenomena (not falling back on the descriptive language of school grammar, which would be familiar to readers but which I believe to be fucked up beyond repair). So there will be a lot of technical talk here; please try to play along, but I don’t think there’s any way to do this right without re-thinking everything from the ground up.)
This is about a perfectly common expression — Who am I kidding? — that went past me in a flash on Facebook this morning but caused me (as a student of GUS — grammar, usage, and style / register) to reflect on the pronoun case in it. On the interrogative human pronoun, appearing here in what I’ll call its Form 1, who, rather than its Form 2, whom.
The pronoun in this expression is the direct object of the verb in the expression, KID, appearing in sentence-initial position (appearing “fronted”) in the WH-question construction of English. There’s nothing at all remarkable about this: in general, both forms of this pronoun are available as syntactic objects (of verbs or prepositions) in the language, differing only in their style / register (very roughly, formal whom vs informal who), with the special case of an object pronoun actually in combination with its governing preposition, which is obligatorily in Form 2:
Who / Whom did you speak to? BUT *To who / ✓to whom did you speak?
So there’s nothing remarkable about Who am I kidding? It’s just informal.
What’s remarkable is the unacceptability of Whom am I kidding? The stylistic discord between the formality of object whom and the informality of the idiom WH-Pro am I kidding? is unresolvable. To put it another way, the choice of the Form 1 pronoun here is part of the idiom. Just like the choice of the PRP form of the verb KID, conveying progressive aspect: Who do I kid? lacks the idiomatic meaning.
