risab;e-1causing or capable of causing laughter
2having the ability,disposition or readiness to laugh
3pertaining to or connected with laughter
A card is what the other students called the fourteen year old Hardy Camp. The teacher had other names for him, but the other students would have said if they lived in an earlier time of Hardy he was a risable chap.
Mrs Hamilton knew of Hardy's reputation for causing an uproar during an examination, so she planned she would invigilate the exam, by walking among the students as they took the exam. If Hardy opened his mouth she would immediately send him out of the room.
On the day of the exam, Mrs Hamilton gave the explanation of her rules, making sure that Hardy knew the expectations. All was going smoothly until Hardy read the word stagnicolous and it turned on his humor button. Poor Hardy could not finish the exam.