You can think of chug as a noise that turned into a word: a low, heavy sound that keeps repeating, chug, chug, chug. It is the sound of something working hard and not working fast. Every use grows out of that beat. A boat moves at the speed of its own engine noise. A business keeps going without ever speeding up. A computer labours and falls behind. Even a throat swallowing a whole drink without a break has the same repeated, rhythm.
From the word’s history
The source records chug as onomatopoeic, made in imitation of a working steam engine, which is exactly the beat the modern uses still carry.
來源記載 chug 是擬聲詞，模仿蒸汽引擎工作時的聲音。今天各個用法帶著的那種重複節奏，正好跟這個來源對得上。
Wiktionary (kaikki extract, source_ref kaikki:3f6f69a190036ae4:614094958), etymology 1 for chug: Onomatopoeic, in imitation of a working steam engine.
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