Synonyms for Ill-considered:

adj.

all (adjective)
ill-judged, improvident, shortsighted.
foolish (adjective)
injudicious, unwise.
frivolous (adjective)
shallow.
ill-advised (adjective)
ill-judged.
ill-considered (adjective)
brash, foolhardy, harum-scarum, hasty, headlong, hotheaded, impetuous, impolitic, improvident, imprudent, impulsive, incautious, indiscreet, injudicious, madcap, precipitant, precipitate, rash, reckless, slapdash, temerarious, unconsidered, unsound, unthinking.
imprudent (adjective)
irresponsible.
impulsive (adjective)
abrupt, extemporaneous, hasty, impromptu, improvised, impulsive, instantaneous, rash, reflexive, spontaneous, unpremeditated, unrehearsed.
rash (adjective)
precipitant, unconsidered.
unprepared (adjective)
haphazard, makeshift, rough-and-ready, unprepared, unrehearsed, undevised, extemporaneous, impromptu, improvised.

n.

brash, careful, foolhardy, harum-scarum, headlong, hotheaded, impetuous, incautious, irresponsible, madcap, mindless, precipitant, precipitate, reckless, shallow, slapdash, temerarious, unconsidered, undiscriminating, unthinking. impolitic, indiscreet, injudicious, unsound, unwise, wise. ill-considered (noun)
ill-judged, improvident, imprudent, shortsighted.

Other synonyms:

brash, mindless, precipitant, shallow, temerarious, unconsidered, undiscriminating, unthinking. foolhardy, harum-scarum, headlong, hotheaded, impetuous, impolitic, incautious, indiscreet, injudicious, irresponsible, madcap, precipitate, unwise. reckless. slapdash. unsound. Other relevant words:
brash, careful, foolhardy, harum-scarum, headlong, hotheaded, ill-judged, impetuous, impolitic, improvident, imprudent, incautious, indiscreet, injudicious, irresponsible, madcap, mindless, precipitant, precipitate, reckless, shallow, shortsighted, slapdash, temerarious, unconsidered, undiscriminating, unsound, unthinking, unwise, wise.