Origin: Germanic Old English prefix
Underline has 6 different meanings across 2 categories:
a line drawn underneath (especially under written matter)
"She used her pen to underline the important dates in her lecture notes so she wouldn't forget them during the exam."
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 editor underlined the phrase "urgent review needed" to signal to the team that it was hyperlinked for immediate access to the report."
give extra weight to (a communication)
"Her gesture emphasized her words"
draw a line or lines underneath to call attention to
"She underlined the key terms in her lecture notes so she wouldn't forget them during the exam."
To draw a line underneath something, especially to add emphasis; to underscore
"She used her red pen to underline the key dates in the itinerary so she wouldn't forget them."
Passing under a railway line.
"The construction crew had to install a temporary barrier before they could safely pass under the railway line during the maintenance window."