Meaty is a term for doing an attack versus an opponent who has been knocked down. Meaty got its name after people first started using big, "juicy", otherwise not very practical attacks with a lot of active frames on waking up opponents to guarantee that it hits and puts them in block stun. Now, players discovered more sophisticated ways to gain and utilise this advantage. The goal is to preemptively attack to make it hit in its later parts of the attack's animation, thus gaining more frame advantage than usual.
What is a Meaty?
After knocking down the opponent, time an attack to hit them with the later active frames of your attack as they wake up. This gives you more frame advantage than usual while disabling most of the opponent's options on wakeup.
The available meaty options are character specific and depend on how the opponent is knocked down. Different knockdowns give you varying amounts of time to prepare a meaty before the opponent recovers, thus you will need to use different meaties for different situations.
To demonstrate the added frame advantage meaties gain, let's compare a meaty versus a non-meaty attack.
Meaty 5K (left) versus normal 5K (right)
Both attackers start attacking at the same time, but notice the meaty attack hits later. As a result, the meaty attack has more frame advantage than the normal attack (compare when the defender is able to start walking backwards).
Some meaties even allow you to recover in time to block reversals from the opponent too!
Sliding Hitbox Meaty
The above example take advantage of the opponent's invincible wakeup animation to have our attack hit later. This same effect can be done by spacing an attack that moves forward, such as doing Slayer's 2D from further away. Players can also use the momentum gained from running forward for the same effect!
Meaty Attack Timeline
Time difference for a normal attack versus a meaty attack. Notice that the amount of blockstun is the same, the key difference is that the meaty attack hits later. Depending on the specific move, an attack that is - on block can turn into + on block!
Moves to Use/Avoid
Moves that inflict lots of blockstun and have little recovery are the best meaty attacks because this allows you to maximize the amount of frame advantage gained. Having said that, they must still be fast enough to hit as soon as the opponent is vulnerable.
One common meaty is a meaty projectile. Ky players often use Charged Stun Edge on oki because it gives him lots of time to run towards the opponent while the opponent is stuck blocking the projectile.
On the opposite end of the spectrum, moves that enter a forced recovery state on hit/block (such as Sol's Riot Stamp or Ragna's Hell's Fang) are generally bad meaties since they do not gain any additional frame advantage from hitting "late" into the move because the recovery will always be the same.
Degrees of Meatiness
Since timing is such a big part of meaties, players often make a distinction for meaties that hit later into the move's active frames versus those that hit earlier. There's no formal terminology for these degrees, so players often use intensifiers and downtoners (very meaty, slightly meaty, etc.)
First Frame Meaty Attacks
While the typical definition of meaty means to hit with the later active frames of an attack, players often refer to attacks that hit an opponent immediately as they wake up as meaty, regardless of whether or not it was with the later active frames or not. A Safe Jump is a common example of this.