shield, buckler, pretext
楯 centers on a protective barrier: a shield or buckler used for defense. The extended sense of 'pretext' comes from using something as a shield in a figurative way, hiding behind an excuse.
楯 combines 木 (tree, wood) with 盾 (shield), suggesting a wooden shield. The phonetic contribution of 盾 is likely, but the exact historical development is uncertain.
A wooden 木 shield 盾: imagine a warrior holding a sturdy wooden shield for protection. That shield can also be a pretext to avoid direct confrontation.
For ジュン, picture a soldier in June using a wooden shield under the hot sun: June -> ジュン, and the shield protects him.
to defy; to disobey; to rebel against; to oppose; to resist
Canadian Shield; Laurentian Shield; Laurentian Plateau
thigh guards (worn underneath armor)
small shield; screen
shield
shield volcano
placoid scale
the humble shield of our Sovereign Lord
gun shield (esp. artillery)