One of the interesting words that came up in my Finnish lessons recently is iloinen [ˈilo̞i̯ne̞n], which means happy, cheerful, glad or merry.

It comes from ilo (joy, happiness, delight, pleasure, love, lover) from the Proto-Finnic *ilo (joy, delight, happiness), and the adjective suffix -inen [source].

Some related words include:

• iloisuus = joyfulness
• iloista = to be happy, glad, cheerful
• iloiesti = happily, merrily, gaily
• iloistua = to become delighted
• iloittelu = frolicking
• iloistutta = to delight, cheer up
• ilottomuus = joylessness
• iloton = gloomy

Related words in other languages include: ilo (joy, happiness) in Ingrian, ilo (fun, joy) in Veps, ilo (joy, elation, happiness, celebration) in Votic, and illu (happiness) in Northern Sami, which was borrowed from Finnish [source].

A related word in Estonian, ilu, used to mean joy delight, happiness or glee, but now means beauty, splendor or ornament [source].

Incidentally, the word ilo (tool) in Esperanto is completely unrelated [source]. It’s a back-formation from the suffix -ilo, which means an instrument or a tool for performing the action of the root. For example, tondilo (scissors) comes from tondi (to shear), and komputilo (computer) comes from komputi (to count, compute) [source].

Ilo can be combined with other suffixes to make words such as ilaro (a set of tools), ilujo (toolbox), and ilarujo (a toolbox for a toolset) [source].

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