You can think of a joker as the one that is not playing by the numbers. In a deck, every card has a fixed place — a number, a suit, a king or a queen — except the one with the on it, which has no value of its own and only means what the game decides. Among people, the joker is the one who steps outside the ordinary run of the conversation to make everyone laugh. The same stepping-outside can be warm or irritating depending on who is watching, and when it is a situation rather than a person, it becomes the part nobody can calculate.
From the word’s history
Wiktionary records joker as joke + -er, literally 'one who jokes'. For the playing card it notes the form may instead come, by alteration, from German Jucker, which also gave the card game euchre its name — and it marks that route as only possible, not certain.
詞源資料寫的是 joke（玩笑）加 -er（做這件事的人），字面就是「開玩笑的人」。至於紙牌上的 joker，同一份資料說它「可能」是從德文 Jucker 變過來的，而 Jucker 也是牌戲 euchre 名字的來源。這條線索本身就標著「可能」，所以這裡照樣寫成可能，不當成定論。
Wiktionary, entry 'joker' (noun), etymology field supplied in packet; source_ref kaikki:3f6f69a190036ae4:441617369
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