匙 is a phono-semantic compound character composed of 是 and 匕. 是 serves as phonetic component and 匕 semantic. In the seal script, 
= 
（correct, exact）
(a spoon)
Even though in a phono-semantic compound character, the semantic component decides the meaning of the character, the phonetic component can still play a subsidiary or auxiliary role to decide the meaning of the character.
For the character 匙, 是 means correct and exact, and 匕 a spoon. The original meaning of 匙 is a small spoon. 匙 expresses an implied meaning that when scooping, one should hold the spoon correctly and exactly, and can’t tilt it.
The extended meaning of 匙 is key. A key should be correct and exact to open the lock, isn’t it true?