冗談ならいざしらず本気でそんなことを言うとは。
- It would be fine if it were a joke, but I can’t believe you said that seriously.

冗談ならいざしらず本気でそんなことを言うとは。 meaning: I'm shocked that you would say something like that seriously, if it were a joke it would be different.

This sentence structure, ～ならいざしらず (or ～はいざしらず), is used to express "if it were...then it would be okay, but...". It implies a contrast or a sense of surprise. The first part introduces a hypothetical situation, and the second part presents the actual situation, often with a negative implication or disapproval. ⚫︎ The general structure is: Noun/Verb (casual form) + なら/ば + いざしらず, (opposite situation/statement). ⚫︎ In this example, 冗談ならいざしらず, the speaker is acknowledging that if the statement were a joke, it would be acceptable. However, since the statement is made seriously (本気で), it is unacceptable or surprising. ⚫︎ It is a fixed expression commonly used in written or formal Japanese. ⚫︎ いざしらず can also be written as いざ知らず.

本気で is an adverbial phrase meaning "seriously," "in earnest," or "for real." It modifies the verb 言う (to say) in this sentence. ⚫︎ It emphasizes the speaker's surprise or disapproval at the gravity of what was said. ⚫︎ 本気 itself means "seriousness" or "sincerity." ⚫︎ This phrase is frequently used in daily conversation and various levels of formality.

The particle と in 言うとは serves as a sentence-ending particle that expresses surprise, shock, or a sense of strong emotion. ⚫︎ It can be thought of as a way of expressing the speaker's reaction to the situation. ⚫︎ The basic function of と is to mark the content of the utterance 言う. In this case, it indicates what the speaker is surprised about. ⚫︎ It often follows a verb in the dictionary form or plain form.

The verb 言う (to say) is conjugated in its plain form in this sentence 言うとは. ⚫︎ In this context, it appears in its dictionary form because it is used before the particle と. ⚫︎ The dictionary form is used to give a description of the utterance. ⚫︎ The particle と then emphasizes the speaker's strong emotion or feeling towards the utterance.

そんな functions as a demonstrative adjective, meaning "such" or "that kind of". It modifies the phrase ことを (thing), pointing out the content that is being referred to. ⚫︎ そんな is derived from the adjectival noun そう (like that, that way) + な (the adjectival form of the copula "da"). ⚫︎ This combination often introduces something that has been said earlier in the conversation or situation. ⚫︎ In this sentence, it refers to the type of thing that is being said.