A caster T readies a spell on their turn, then an enemy caster E targets them with feeblemind. T passes the Constitution save against the 4d6 psychic damage to maintain concentration, but fails the Intelligence save and suffers the greater effects of feeblemind.

As I understand it, readying a spell casts the spell but does not release it.

Feeblemind says of the target, in part:

It can't cast spells, activate magic items, understand language, or
communicate legibly by any means. However, it can identify allies, and
follow and protect them.

Since T has already cast the spell while readying it, does T lose the readied spell when they are affected by feeblemind?

The readied spell is not lost

The description of the Ready action (PHB p. 193) says:

When you ready a spell, you cast it as normal but hold its energy, which you release with your reaction when the trigger occurs. To be readied, a spell must have a casting time of 1 action, and holding onto the spell's magic requires concentration. If your concentration is broken, the spell dissipates without taking effect.

So you cast the spell first, and afterwards you are simply "holding" its energy. Since feeblemind does not specify that the target loses the ability to "hold" or "release" the energy of spells (only that they cannot "cast" them), there is nothing stopping the caster from doing those things.

RAW: No, you can maintain concentration - but you might have to do an extra DC 10 Constitution saving throw to do so

As concentration is a Constitution saving throw, which is not affected by feeblemind, there is no reason why you should not be capable to keep concentrating on the spell.

The rules on concentration specify the following reasons for loosing concentration

• Casting another spell that requires concentration. [...]




• Taking damage. [...]




• Being incapacitated or killed. [...]

The DM might also decide that certain environmental phenomena, such as a wave crashing over you while you're on a storm-tossed ship, require you to succeed on a DC 10 Constitution saving throw to maintain concentration on a spell.

The first 3 reasons do not apply, and the fourth is at your DM's discretion.

Similar situations such as polymorph also do not make you lose concentration. The druid's Wild Shape feature actually explicitly mentions that you can maintain concentration:

You can't cast spells, [...] Transforming doesn’t break your concentration on a spell you’ve already cast, however, or prevent you from taking actions that are part of a spell, such as call lightning, that you’ve already cast.

So, the RAW says that no, you can keep concentrating. Though your DM might force an extra Con save.

As a DM, I would allow you to keep concentrating under the condition that you use the spell the first chance you get.