clabber vs thicken

clabber

verb

To sour or curdle.

noun

Sour or curdled milk.
Wet clay or mud.

thicken

verb

To strengthen; to confirm.
To become thicker (in the sense of wider).
To become thicker (in the sense of more viscous).
To make thicker (in the sense of more viscous).
To make thicker (in the sense of wider).
To make more frequent.

How often have the words clabber and thicken occurred in a corpus of books? (source: Google Ngram Viewer )