By the way introduces something slightly off the main subject. A small aside, dropped in as you pass.
It usually opens a sentence, and it politely marks what follows as extra rather than central. That is often a fiction, since the thing introduced by the way is frequently the real reason for the conversation, and everybody knows it. Written English shortens it to BTW in messages and email. Incidentally is the formal equivalent and does exactly the same job with more syllables.
1phraseby the wayUsed to introduce a remark not directly connected to the main subject.
“By the way, did you ever hear back about the flat?”
First appeared in the 10th century with literal meaning "by the side of the road", from Old English weġ (which became "way") meaning "road". Soon afterward (circa 1000) it came to mean "during one's journey". The figurative meaning developed in the mid-16th century, with the first instance of the modern meaning being from 1614.
Etymology from Wiktionary, CC BY-SA
Practised in A Man Called Ove, Small Talk & Everyday Phrases
First appears in chapter 17 of A Man Called Ove.
Practise this word