Similar word to insubstantial:

aerial, airy, chimerical, decrepit, ephemeral, false, fanciful, feeble, flimsy, fly-by-night, fragile, frail, idle, illusory, immaterial, imponderable, incorporeal, infirm, intangible, metaphysical, petty, poor, puny, slender, slight, tenuous, thin, too little too late, unrea