(attributive) on call

Available to be summoned.

(idiomatic) Holding starkly opposite views on an issue.

At the site of a job, or at the location of the customer of a product or service, as opposed to at the vendor's factory or office.

(informal, idiomatic) Enthusiastic for. / (informal, idiomatic) Knowledgeable about. / (informal, idiomatic) Skilled at.

(transitive) to turn a switch to the "on" position in order to start or enable a device / (idiomatic) to change one's expression or appearance as if by turning a switch

(informal, Britain, Australia) Unacceptable or impossible.

(baseball) The circle drawn or painted on the field where the next batter to hit waits.

(slang) To strike an opponent heavily and repeatedly in a fight. / (slang) To beat heavily on anything.

(slang) Alternative form of whale on