To mark items on a list (with a checkmark or by crossing them out) that have been chosen for keeping or removal or that have been dealt with (for example, completed or verified as correct or satisfactory); to tick off (UK), tick (UK), cross off, strike off.
To carry out a decision recorded through the process of checking off an item on a list, including dealing with it, removing it from the possible choices, or removing the not checked items from further consideration.
To deduct union members' dues from their wages.
To emphasise or stress something
To draw a line underneath something, especially to add emphasis; to underscore
A line placed underneath a piece of text in order to provide emphasis or to indicate that it should be viewed in italics or (in electronic documents) that it acts as a hyperlink.
The character _.
Passing under a railway line.