Guesswork

Mere days ago, I name-checked Mrs. Malaprop, a character from the Sheridan play "The Rivals. (Her name comes from French mal à propos, "inappropriate", and she's given her name to a usage error in which a word is replaced by a similar-sounding one: "She stuck to him like a leash", for example.) And now, look--here's one in the wild!

Let's see if you can spot the malapropism. When I first read this link from Boingboing, I just stared at it, because I couldn't figure out if the writer was seriously wrong, or was making some kind of joke, or something I didn't even have a name for.
Designer Wong makes engagement rings that can kill you. The razor-sharp diamond point is set into the ring so it can’t get knocked out when you smash someone’s face in, and the edges of the ring are really soft so it won’t cut into your skin during the pounding. It’s romantic because it means, “Will you marry me” but it also means, “I can’t always be there to protect you so if some jerk won’t stop bothering you, puncture him with this.” The diamond sharp edge will also cut skin down to the bone (with a minimum 1 karat stone - but the larger the better). Or it may simply be used to tag hard surfaces, like cars and windows for S.O.S. messages or that last will and estimate when pen or paper (or lawyers) aren’t conveniently around.

The ring is a good joke. The phrase "last will and estimate"? Not so much. I mean, it might be a joke and I just don't get it, but it's a very plausible malapropism for "last will and testament".