Typewriters let you backspace _over_ what was already there, so we just typed the word, used backspace to get the beginning of the word, and then typed undersco

Typewriters let you backspace _over_ what was already there, so we just typed the word, used backspace to get the beginning of the word, and then typed underscores until the word was effectively underlined. In fact, you could double underline a word by moving half a line down (that was a thing) and then create the underline with equal signs.