There are two main etymological versions. The first (roughly speaking) - from "cracklings" (that is, "fried fragments"), "squirt" (onomatopoeia, imitating sounds when frying). Wed also "shkvara" (metal slag, waste from smelting, forging), Czech. škvár (slag, old stuff, junk). That is, "getting greasy" is literally "getting dirty", "getting dirty", The second (which I like better) is from the Old Russian basis "skvr-" (compare Old Russian "skvrna" - " abomination, evil "; goes back to the Greek. skor -" dirt "). Also, it turns out, it comes down to "dirtying".
Get dirty, about a rooster, i.e. descend. I drank from a leaky cup, I got greasy, I became a rooster. It is likely that the grease is dirt, inferior, omitted.
This originally came from a prison environment. To be buried - to come into contact with the lowered (rooster), his things or place on the bunk, in other words - to be defiled. Zashkvar is a shame / shame.