It means to have sex with a person or to go to second base at least or actually anywhere with just a person. Even when they fancy a person or is thinking about getting with the person or tryna get with the person.
"YEOW (yeow is said at a loud high-ish pitched tone), __________ (followed by the name of the person) gettin the log in (said the same as yeow but at a lower octave of the pitch), nice one (also said the same as get the log in)"
"YEOW, conor gettin the log in, nice one"
or "the log is in"