The word is used, in modern English, to mean roughly this was meant to be — usually about a meeting, usually warmly.

The root means he divided.

Arabic q-s-m, qasama, to divide. From it comes qisma — division, lot, portion, destiny — and the image underneath is arithmetic rather than mysticism. Kismet is not the universe intending something for you. It is your share of a thing that was divided up.

Which gives it a rather different temperature. Meant to be implies an intention and, faintly, an intender. Your portion implies an allocation — a fixed quantity, distributed, with your name on some of it and not on the rest. It is closer to bookkeeping than to romance, and the earliest English uses reflect that: the 19th-century citations are fatalistic and resigned, the word of someone accepting an outcome rather than celebrating one.

The route into English is also worth getting right, because it is regularly reported wrong. Not Arabic to English. Arabic → Ottoman Turkish → English, arriving in 1834 from Turkish usage — which is why the English form keeps the Turkish shape. A novel titled Kismet in 1877 pushed it into general currency; Kipling was using it by 1887, and Fitzgerald wrote of fickle Kismet by 1920.

The warm sense is the newest layer, and it inverts the original.

And the contrast with serendipity is unusually clean, because the two words disagree about where the good thing came from. Serendipity requires you to have been sharp enough to notice something you weren't looking for — it is earned, in a specific and demanding way.

Kismet requires nothing at all. It was already yours. Your portion was divided out before you arrived, and noticing it changes nothing about the size of the share.