This is the 9493rd most frequent Russian word.
усику
The word "усику" is the dative or prepositional singular form of "усик," which means "a small mustache," "an antenna (of an insect)," or "a tendril (of a plant)" in English.
In this sentence, "усику" is used as a dative case of the noun "усик", used as a proper name, likely referring to someone or something nicknamed or personified as "usik".
Усику подарили новый мотоцикл.
Usiku was gifted a new motorcycle.
Here, "усиком" is an instrumental case of the noun "усик", indicating "antenna" or a small sensory appendage of an insect.
Я видел пчёлку с крошечным усиком.
I saw a little bee with a tiny antenna.
In this context, "усиком" represents the instrumental case of the diminutive form of "ус", meaning "moustache", showing light humor or affection.