よほど・よっぽど
very, (must have been) so; much (more)
よほど + adjective / state · よほどの + Noun
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よほど has three common uses.
Must have been very: a guess about how strongly someone felt, based on what they did: よほど疲れていたのだろう、彼はすぐに寝てしまった, "he must have been really tired, because he fell asleep straight away". It often pairs with のだろう or んだね.
Much (more), in comparisons: 歩くより、タクシーのほうがよっぽど速い, "a taxi is much faster than walking".
よほどの + noun is "something serious": よほどのことがない限り、休まない, "I won't take time off unless something serious happens".
よっぽど is the more emphatic, spoken version. With たい or ほうがいい, it also means "I've half a mind to": よっぽど帰ろうかと思った, "I very nearly went home".
Examples
よほど疲れていたのだろう、彼はすぐに寝てしまった。
He must have been really tired, because he fell asleep straight away.
よほどのことがない限り、休まない。
I won't take time off unless something serious happens.
よほどうれしかったのか、彼女は泣き出した。
She must have been really happy, because she burst into tears.
歩くより、タクシーのほうがよっぽど速い。
A taxi is much faster than walking.
よほどお腹が空いていたんだね。
You must have been really hungry.