[MarkDean]

IMPs all white.

You are second seat:

(1♥) 2♦ (x) P
(2♥) P (3♥) 4♥
RHO asks what 4♥ means, how do you answer?

[Apollo81]

I answer "no agreement"

[mtvesuvius]

I hope RHO doubles

[dicklont]

I answer: "probably misclick, will you let him undo?"

If I must still bid after this I will have to look at my hand first.

Partner could have shown support right away with a cuebid. He did not do that.
I figure that he holds the black suits. When his hand is really extreme there might be some merrit to his bidding, but for now I don't like it.

[matmat]

"p missed his optometrist appointment last week, must have had a diamond mixed in with his hearts"

[rogerclee]

Nothing seems very plausible, I would guess that partner has a spade stack and was hoping to be able to double some number of spades.

[Fluffy]

partner obviously has a big diamond fit, and was willing to let 2♦ play dobled

[neilkaz]

In general, many players panic extremely when they see a bid they don't understand. We will assume that this is a not a nonsensical missclick if played online. Anyhow, whether online or not, I answer no agreement since we don't gave one.

Why didn't PD support ♦ on his first turn or xx ? Who knows ? and do we really care why he didn't since if he has ♦ support he can bid them later. Most likely PD has very large number of black cards and a weakish HCP count. Unless I have garbage in ♠ I'll bid 4♠ here. If my ♠ support is poor and I can stand ♣ I can bid 5♣. If PD doesn't like a black suit bid, he'd better be prepared to reatreat back to 5♦. And, if I can tolerate ♣ and can't stand ♠but also am happy with minimal ♦ support I can punt it back to PD with 4NT and if he think's that's Blackwood, I may want a new PD.

Anyhow..don't panic, explain it as no agreement since you don't have one and do your best to give PD an option below 5♦.

[Codo]

I answer "no agreement", this part is easy.

I think: Maybe he is looking for a new partner- no, it was surely a misclick.
There is nothing what makes sense. Can he have a strong hand for diamonds and hope to play in 2 Diamond doubled? Crazy idea. And even then had 5 ♦ been enough to show a strong hand with diamonds.

[helene_t]

I think Neilkaz hit it. Partner could have a weak hand with 5♠ 6♣ or similar

Alternatively a very strong hand with spades and diamond tollerancce which he could show before because 2♠ would be NF.

[peachy]

Answer RHO's question. "We have no agreement". It would be illegal to answer by giving your guess what it "could be" if this auction has never happened before.

If it has happened before but you have not specifically made agreements, then tell opponents something like "last time this happened, he had *this hand* but we have not discussed it. "