Mr. Collins made his declaration in form.

diffidence: the quality of being shy and not confident of your abilities

eg. Having resolved to do it without loss of time, as his leave of absence extended only to the following Saturday, and have feelings of diffidence to make it distressing to himself even at the moment, he set about it in a very orderly manner, with all the observance, which he supposed a regular part of the business.

injunction: an official order given by a law court, usually to stop someone from doing something

eg. Elizabeth would not oppose such an injunction—and a moment's consideration making her also sensible that it would be wisest to get it over as soon and as quietly as possible.

You can hardly doubt the purport of my discourse, however your natural delicacy may lead you to dissemble; my attentions have been too marked to be mistaken.

dissemble: to hide your real intentions and feelings or the facts

purport: the general meaning of someone's words or actions

temper: to make something less strong, extreme, etc.

eg. And your wit anc vivacity, I think, must be acceptable to her, especially when tempered with the silence and respect which her rank will inevitably excite.

melancholy: sad

eg. I could not satisfy myself without resolving to choose a wife from amoung his daughters, that the loss to them might be as little as possible, when the melancholy event takes place.

hitherto: until now or until a particular time

eg. If what I have hitherto said can appear to you in the form of encouragement, I know not how to express my refusal in such a way as may convince you of its being one.

manifold: many and of several different types

eg. And you should take it into further consideration that, in spite of your manifold attractions, it is by no means certain that another offer of marriage may ever be made you.

…，whose behaviour at least could not be mistaken for the affectation?and coquetry of an elegant female.

affectation: behaviour or speech that is not sincere

coquetry: behaviour in which someone tries to attract attention by pretending to be sexually interested in people, in a pleasant but not serious way.