all, entirely, altogether, completely, use up, run out of, exhaust, befriend, serve
悉 centers on totality and completeness: all of something, entirely, or using something up until it is exhausted. The less common senses of befriending or serving are likely extended uses where one gives oneself completely.
悉 combines 釆 (a footprint or distinguishing mark) with 心 (heart/mind), suggesting a thorough, all-encompassing knowledge or complete understanding. The exact historical development is uncertain, but the component combination points to completeness.
The top 釆 looks like a footprint or a mark, and the bottom 心 is the heart. Imagine your heart leaving its mark on everything, covering it all completely until nothing is left.
For シツ, picture a sheet of paper that is completely filled with writing: sheet -> シツ, and the entire sheet is used up.
altogether; entirely
Very Sincerely Yours
Siddham (characters)
Siddham studies
Siddham script
all-or-none law
all; wholly; entirely; without exception
all things have the Buddha nature
full-count survey; complete enumeration; census
having a complete knowledge (of); knowing everything (about); knowing thoroughly; being well acquainted (with)
all things have the Buddha nature
Susiddhikara Sutra