Kyle is right. It can also have a related, idiomatic reason: “to consider to task to be done” or “to consider someone no longer to be available.” The metaphor is that you have a list of, say, things to do, and when you get one done, you cross it off the list by putting a line through it. Examples: “We’re done with task X. We can put a line through it and move on to task Y.” “We were considering 5 candidates for the job, but Sue withdrew from consideration. We can put a line through her name.”

It means to draw a line through something. If you make a mistake when writing, you can put a line through the words you have written to show that they are wrong. For example “ w̶r̶o̶n̶g̶”. If you have drawn a picture or diagram, you can put a straight line through the middle of it to show it is incorrect.