The 口 component is sometimes used to indicate an abstract meaning taken from a concrete object. Here, the concrete object "shield" evokes the abstract meaning "firm, solid".
Character Evolution
Oracle Bone~1250-1000 BCOracle Bone~1250-1000 BCBronzeEarly Western Zhou~1000 BCBronzeMid Western Zhou~900 BCBronzeMid Western Zhou~900 BCBronzeLate Warring States~250 BCSealWarring States (Chu)475-221 BCSealWarring States (Chu)475-221 BCSealShuowen~100 ADSealShuowen~100 ADClericalQin221-206 BCClericalWestern Han202 BC-9 ADClericalEastern Han25-220 ADRegularModern