Learn English – the origin of the phrase “I’ll take a raincheck”

Nobody really knows.

There's no agreed derivation of the expression 'hunky-dory'.

It is American and the earliest example of it in print that I have found is from a collection of US songs, George Christy's Essence of Old Kentucky, 1862.

We do know that 'hunky-dory' wasn't conjured from nowhere but was preceded by earlier words, i.e. 'hunkey', meaning 'fit and healthy' and 'hunkum-bunkum', which had the same meaning as 'hunky-dory'. 'Hunkey' was in use in the USA by 1861, when it was used in the title of the Civil War song A Hunkey Boy Is Yankee Doodle. 'Hunkum-bunkum' is first recorded in the US sporting newspaper The Spirit of The Times, November 1842.

I found the first entries during history, not the exact origin of the expression, unfortunately. They are around the 17th Century.

by the by (earlier by a by, on or upon the by): by a side way, on a side issue; as a matter of secondary or subsidiary importance, incidentally, casually, in passing. Obs. or arch. Also in predicative or complemental use (quasi-adj.):
Off the main track, away from the point at issue, of secondary importance, incidental.

Here are some examples:

• 1615 W. Hull Mirr. Not intentionally, but accidentally (as we say) vpon the bye.
• 1627 Hakewill Apol. It led them some other way, thwarting and upon the by, not directly.
• 1642 Fuller Holy & Prof. St. They had something‥in the favour of Friers, though brought in only by the by.
• 1678 Butler Hud. All he does upon the By, She is not bound to Justifie.    
• 1740 J. Clarke Educ. Youth (ed. 3) Let it be done sparingly, and by the bye.

Source: OED