One way to hold this word: -er names the one that does the job. Start with a person doing the kissing, then let the name slide down onto the part of them that actually does it, and then out onto the whole face around that part. Each step keeps the same small idea and points at something new with it. By the last step the kissing has dropped out of the meaning entirely, and only the place is left — which is also why an archer can call a button on a bowstring a kisser: it is the thing that meets the lips.