IshGen 118:8 :ܩܒܘܬܐ ܩܪܗ݀ ܡܛܠ ܐܣܟܝܡܐ ܕܬܘܩܢܗ݀ ܒܕܠܘ ܓܠܝܠܬܐ ܗܘܬ܂ ܘܬܘܒ ܡܛܠ ܩܒܝܒܘܬܗ݀‏ : he called it "ark" because of the shape of its construction, in that it was not round, and also because of its vaultedness
See the entire entry for qbybw n.f. curvedness