Entry 1 sense Webster, 1913 Semipenniform /(sĕm`ĭ*pĕn"nĭ*fôrm)/ · Sem·i·pen·ni·form · IPA 01 a. Half or partially penniform; as, a semipenniform muscle. 1. Half or partially penniform; as, a semipenniform muscle.(Anat.)