Від чого походить слово "спасибі" (для вираження подяки)?
Both Ukrainian спасибі and Russian спасибо come from спаси Бог, which means "save [you] God" (спаси is an imperative mood for спасати "to save", Бог means "God"). After the centuries of usage these words got merged into single word, which got later shortened.
We also say дякую for expressing gratitude, which is a 1-st person form of the verb дякувати "to thank". I.e. we could say я дякую тобі, which literally means "I thank you", but we usually omit both я "I" (because it's implied when using 1-st person form of a verb) and "you" (because it's implied by context).
For references see: