"Have it off with" is an informal phrase that means to have sexual intercourse with someone. However, there are many possible synonyms for this term, ranging from the mildly suggestive to the extremely vulgar. Some common alternatives include "hook up with," "get laid," "make love to," "sleep with," "do it with," "have sex with," "get it on with," and "fool around with." These terms can vary depending on the context and the speaker's intention, but in general, they all refer to the act of engaging in sexual activity with another person. It's important to use these terms responsibly and with respect for everyone involved.