an expression of appreciation at someones arrival; welcome
日本へようこそ！
Nihon e yōkoso!
Welcome to Japan.
私たちの家へようこそ。
Watashitachi no uchi e yōkoso.
Welcome to our home.
↑ 1998, NHK日本語発音アクセント辞典 (NHK Japanese Pronunciation Accent Dictionary) (in Japanese), Tōkyō: NHK, ISBN 978-4-14-011112-3
Etymology
yoku koso → you koso → yoːkoso. From the adverb 良く(yoku, “well”) and the emphatic particle こそ(koso). The medial -k- drops out and the resulting -ou- elide into a long vowel.