Synonyms for flimsy

Most common

1. Weak — likely to bend or break under pressure; easily damaged; frail, unsubstantial. Of clothing, very light and thin

2. Silly — of a person, lacking depth of character or understanding; frivolous, superficial

3. Ill-founded — of an argument, explanation, etc., ill-founded, unconvincing, weak; also, unimportant; paltry, trivial

4. Sickly — of a person, their physical makeup, or their health, delicate, frail

Other

thin, unconvincing

adj. 1
Likely to bend or break under pressure; easily damaged; frail, unsubstantial. Of clothing: very light and thin.
adj. 2
Of an argument, explanation, etc.: ill-founded, unconvincing, weak; also, unimportant; paltry, trivial.
the flimsiest of theories
adj. 3
Of a person: lacking depth of character or understanding; frivolous, superficial.
adj. 4
Of a person, their physical makeup, or their health: delicate, frail.

Use in printed books, per decade from 1850 to 2019.

4 letters: fine¹, poor¹, weak¹, thin¹, puny²
5 letters: light¹, cheap¹, silly², shaky², frail¹, inane², gauzy
6 letters: futile², feeble², weakly², trashy², shoddy², sickly²
7 letters: shallow², fragile², foolish², brittle², tenuous³, rickety³, flighty², puerile²
8 letters: delicate³, trifling², gossamer³
9 letters: frivolous³, unfounded³, breakable³, unserious
10 letters: groundless²
11 letters: transparent³
12 letters: unconvincing⁴
13 letters: insubstantial⁴
15 letters: unsubstantiated⁶

The small number after each word is its number of syllables (puny² = 2 syllables).