Think of the small turn a sentence makes when it stops telling and starts asking. 'He is ready' is flat and finished. 'Is he ready?' leans forward and waits for someone else to fill the gap. Interrogative is the label English puts on anything wearing that forward-leaning shape: a whole sentence, a single word like 'who' or 'what', and, in written English, even a face or a voice that waits for an answer without asking out loud.
From the word’s history
Wiktionary records the word as coming from Late Latin interrogativus, equivalent to interrogate plus the adjective ending -ive — literally 'to do with asking'. Every modern use still sits on that one job.
Wiktionary 記載 interrogative 來自晚期拉丁文 interrogātīvus，等於 interrogate（盤問、提問）加上形容詞字尾 -ive，字面就是「跟發問有關的」。今天的每一個用法都還踩在這個工作上：不管貼在句子、單字還是眼神上，被貼上的那個東西都在發問。
Wiktionary entry 'interrogative' (packet source_ref kaikki:3f6f69a190036ae4:1188273240:14431): 'From Late Latin interrogātīvus, equivalent to interrogate + -ive.'
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