/ ˌɪnsəbˈstænʃl; NAmE / adjective
1.
not very large, strong or important :
an insubstantial construction of wood and glue
an insubstantial argument
2.
( literary ) not real or solid :
as insubstantial as a shadow
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WORD ORIGIN
early 17th cent.: from late Latin insubstantialis , from in- not + substantialis (from substantia being, essence, from substant- standing firm, from the verb substare ).