This is the 21st most frequent Estonian word.
sest
"Because."
In this sentence, "sest" is used as a conjunction meaning 'because', introducing the reason for being late.
Ma jäin hiljaks, sest ma magasin sisse.
I was late because I overslept.
Here, "sest" serves as a causal conjunction, explaining the reason for not coming.
Ta ei tule, sest ta ei tunne end hästi.
They are not coming because they are not feeling well.
In this case, "sest" links the inability to go with the reason being bad weather.