etymology - Why does "smashing" mean "very good"?

The slang smash means a great success and was first used in the world of entertainment.

A smash

If a musical, play, or film is a smash, it means it is hugely successful. Whenever a show or movie rakes in more money than its leading predecessor we hear the claim: breaks box office record. In this instance break, which is synonymous with smash, has positive connotations.

Later in the world of rock'n'roll and pop we hear of songs hitting the charts and being called smash hits. So it's not difficult to see how a smash (hit) could eventually evolve into the adjective smashing [= great] or vice versa.

Dictionary.com says,

smash hit an outstanding success, as in
She was a smash hit in the role of the governess,
His first book was a smash hit but this one isn't doing well. [c. 1920 ]

Merriam-Webster Dictionary tells us

smash _^noun: someone or something that is very successful or popular
- the sound made when something hits a surface very violently
- a hard downward hit in tennis or other games

Origin of SMASH
perhaps blend of smack and mash
First Known Use: 1725

smash _^verb: to break (something) into many pieces : to shatter or destroy (something)
- to hit (something) violently and very hard
- to hit (a ball) downward and very hard in tennis and other games
First Known Use of SMASH 1764

smashing _^adjective: very good or impressive
1: that smashes : crushing a smashing defeat
2: extraordinarily impressive or effective a smashing performance
First Known Use of SMASHING 1825

Etymonline states clearly that smashing meaning "pleasing, sensational" is from 1911. While smash meaning "great success" is from 1923

It's Not Tennis

I don't believe smashing derives from the famous tennis shot. In tennis a smash is a ball hit with such force, speed and power that can leave the opponent "defeated", inasmuch as the player fails to return the shot. Smash is onomatopoeic, it describes the sound of the ball hitting the ground, metaphorically it "shatters" the court. From the website, The History of Tennis—The Origins of Tennis, we learn

1880 ▪ BIRTH OF OVERHEAD SMASH ▪ The overhead smash was introduced
into the game for the first time in the history of tennis by the
Renshaw brothers in Wimbledon. They would dominate Wimbledon for a
decade, winning all but 1880 and 1887 championships between them in
the history of tennis.

Smashing

The 1919 edition of the Oxford English Dictionary [link] has no record of smashing meaning sensational, wonderful or excellent. Instead its authors defines it as being a verbal substantive (vbl.sb) formed on the meaning of the verb smash

Smash v.¹ [Probably imitative: Norw. dial. smaska to crush, slaa i smask to knock to smash (Ross).]
2. To break (anything) in pieces violently; to dash to pieces; to crush, shatter, or shiver.
3. To dash or fling (anything) with noise and violence; to batter; to cause to strike hard.

Smashing vbl.sb. ¹ [f. Smash v.¹] 1. The action of SMASH v.¹ in various senses

The noun smash is described as being dialect or colloquial: A hard or heavy blow

There's no mention of it meaning a success. However, the OED 1919 edition has some information on the noun smasher