laetus

Laetus, laeta, laetum is an ordinary first and second declension adjective: adverb laete, comparative laetior, superlative laetissimus, noun laetitia. Latin often uses it predicatively where English wants an adverb, so laetus rediit is better rendered "he came back happy" than "he came back happily".

The older, earthier sense is worth having. Of soil, crops and cattle laetus means fat, rich, thriving, and that is what Virgil's laeta arva are. Medieval Latin even coined laetamen for manure, which tells you exactly what makes a field cheerful. One trap: laetus is unrelated to letum, "death", despite the near-identical sound once the ae flattens in later pronunciation, and scribes did confuse the two spellings.