The original meaning of the word yester is simply ‘previous’.
The Old-English word ġiestran referred specifically to the previous day; nowadays referred to as ‘yesterday’. In the transition from Old-English to Middle-English the compound meaning was lost and drawing upon Proto Germanic languages yister and later on yester were generally used as the modern word previous.
Why this word?
Yester is a forgotten adjective, but moreover- it is a forgotten prefix. We are all familiar with the more-than-common compound word yesterday, but do we ever use yesternight? This word too originates from Old-English ġiestranniht and is simply a more special way of referring to last night. Yestereven is another way of referring to the last evening. We can also say yesteryear to refer to the previous year and yesteryore to refer to the prehistoric age.
Yestersol is the old way of saying yesterday, compound of the already discussed ġiestran and sol, the old word for sun. This one also stands for a funny story; NASA researchers did not feel comfortable referring to a Martian day (a day on Mars) simply as ‘day’ as it averagely lasts 37 minutes and 39 seconds longer than our earthly days (24 hours). In the previous decade NASA researchers had adopted sol to represent a Martian day, and consecutively yestersol to stand for the Martian previous day.
How to use yester?
Making close acquaintance with this wonderful prefix does not only enrich your speech with yesternight, yestereven, yesteryear and yesteryore, but it also gives you the simple, natural and beautiful adverb yesterly, elegantly standing for ‘previously’.