1. 无意中说;随口说
    If you throw in a remark when having a conversation, you add it in a casual or unexpected way.

2. 免费赠送;额外奉送
    If a person who is selling something throws in something extra, they give you the extra thing and only ask you to pay for the first thing.

1. to insert between other elements

    e.g. She interjected clever remarks

    Synonym: interjectcome ininterposeput ininject

2. give up in the face of defeat of lacking hope
    admit defeat

    e.g. In the second round, the challenger gave up

    Synonym: drop outgive upfall by the waysidedrop by the waysidethrow in the towelquitchuck up the sponge

3. add as an extra or as a gratuity