(as) cross as a bear

Exceptionally irritable, annoyed, or grumpy. John is always cross as a bear first thing in the morning; it's best not to even talk to him until he's had his coffee. Janet was as cross as a bear when she found out she had missed her connecting flight.

Grumpy, ill-humored, annoyed, as in Stay away from Claire; she's cross as a bear this morning. Unlike the earlier cross as two stocks, this survives even though the adjective cross for "ill-tempered" is otherwise not used much in America. It is sometimes amplified as cross as a bear with a sore head. [Early 1700s]

Irascible, peevish, vexed, cantankerous. This simile dates from the late eighteenth century and has survived as a cliché on both sides of the Atlantic even though the adjective cross for “angry” is heard less frequently in America. Why a bear should be considered particularly prone to anger is not entirely clear, and indeed, over the years writers have amplified the description somewhat—for example, “cross as a bear with a sore head,” “mad as a beaten bear,” “grumbled like a bear with a sore ear.”