Kunai (苦無) (translates to “without any problem”) is a ninja tool to dig, serve as holders for torches or a short sword and shuriken. A large Kunai called “daikunai” (Large trowel; 大苦無) and a smaller one “shokunai” (small trowel; 小苦無).
Tobikunai (flying trowel; 飛苦無) had a kaginawa (rope with hook; 鉤縄) or similar solid item in the end to be thrown. There were also tobikunai with long handles, where a drill or small nail could be fixed and could then be used as both tools and weapons. There were even times when a stack of dry grass, twigs or the like could be tied on it to be used in Katon (escape using fire; 火遁).
Kunai was thus a tool for breaking in that could also be used as a weapon.
From: Sengoku Ninpo Zukan (1978) by Masaaki Hatsumi in free translation
 For the sake of consistency in speech, the term “trowel” will be used here to refer to the kunai as it shares many of the same uses as a trowel. – Luke Crocker