put aside, put down, discard
寘 centers on placing something down or aside, often with a sense of finality: putting something away, setting it down, or discarding it.
寘 combines 宀 (roof, building) with 真 (true, real). The exact historical development is uncertain, but it likely expressed the idea of placing something securely inside a building.
Under a roof 宀, you place something true 真 and real down for safekeeping, or discard it if it's no longer needed.
For シ, imagine you see a discarded item under the roof and say 'Sheesh!' (シー) at how carelessly it was put aside.