This wiki article describes Ajanbahu as
a person, whose length of arms is so that his finger touch his knee
It also claims that many Hindu Gods are Ajanbahu.
Is there any deeper spiritual significance to this term?
Ajanubahu is a person who hands are long that it reaches the knees of that person when that person stands.
I believe "Ajanubahu" explains a great personality of extraordinary people physical appearance and this is the deeper meaning in this word.
जानु = knee | बाहु = arm | आजानुबाहु = one whose arms reach up-to his knees
Shri Rama is also called Ajanubahu, since his arms reached upto his knees.
Sri Rama’s limbs have come for description often – Ajanabahu. The Ramayana describes Sri Rama in a verse thus:
Sri raghavam dasarathatmajamaprameyam
sitapatim raghukulanvaya ratna dipam
ajanubahu - araivinda dalayataksham
ramam nisachara vinasakaram namami
Another story in the Mahabharata says when Uttara, Abhimanyu’s wife was with child, and Ashwattama sought to kill the foetus, it was Krishna’s mercy and divine intervention that saved the baby. There in the womb, yet-to-be-born Parikshit (originally christened as Vishnudatta – gift of Vishnu) saw the divine form of the Lord, golden hued and with arms so long that reached his knees.
All these naamas (Pleasant) starting from Suvrana (Gold) are a glorification of the beauty of Bhagavan (God), as having beautiful limbs that are pleasing to the eyes as well as which perform actions that are beautiful/pleasing.