Sentence:  Rounding up the animals in the misty paddocks, with the blackbirds singing as the morning whitened, he felt <WSD>hilarious</WSD>, light-headed. He'd clap the cows on their rumps and shout "Come along, there! Come along there, me Irish darlint." 
Meaning of the word marked with <WSD> tags:  Full of hilarity; merry.