let someone have it, Informal. to attack or assault, as by striking, shooting, or rebuking: The gunman threatened to let the teller have it if he didn't move fast.
Origin: before 900; Middle English leten,Old English lǣtan; cognate with Dutch laten,German lassen,Old Norse lāta,Gothic lētan; akin to Greek lēdeîn to be weary, Latin lassus tired. See late
Usage note Let us is used in all varieties of speech and writing to introduce a suggestion or a request: Let us consider all the facts before deciding. The contracted form let's occurs mostly in informal speech and writing: Let's go. Let's not think about that right now. EXPANDPerhaps because let's has come to be felt as a word in its own right rather than as the contraction of let us, it is often followed in informal speech and writing by redundant or appositional pronouns: Let's us plan a picnic. Let's you and I (or me) get together tomorrow. Both Let's you and me and Let's you and I occur in the relaxed speech of educated speakers. The former conforms to the traditional rules of grammar; the latter, nonetheless, occurs more frequently. See also leave1.