For the opposite of
indent, I would use
unindent. From Wiktionary:
un- + indent; Originated in the 1980s when computers made
it trivial for anyone to move text around on a page.
unindent (third-person singular simple present unindents, present
participle unindenting, simple past and past participle unindented)
remove the indentation; to move a block of text closer to the left
I had to unindent the first line of each paragraph so that my
essay would fit onto one side of paper.
According to the Online Etymology Dictionary, indent comes from:
early 15c., indenten/endenten "to make notches; to give (something) a
toothed or jagged appearance," also "to make a legal indenture," from
O.Fr. endenter "to notch or dent, give a serrated edge to," from M.L.
indentare "to furnish with teeth," from in- "into, in, on, upon" (see
in- (2)) + L. dens (gen. dentis) "tooth" (see tooth). Related:
Indented; indenting. The printing sense is first attested 1670s. The
noun is first recorded 1590s, from the verb. An earlier noun sense of
"a written agreement" (late 15c.) is described in Middle English
Dictionary as "scribal abbrev. of endenture."
So indenting is like to putting a "notch" in the source code. To remove that notch, you undo the indent: unindent.