The idea contains within the image for gemini 23, has to do with a time of passage. Like fledglings, one may have all the natural instinct in the world to fly, but one may not be ready to leap into the open air.
It is not as if Gemini 23 individuals are fearful or eternally unprepared. It is just that the experience of stepping across a threshold into a new realm is likely to be common.
When interpreting the Sabian symbols, always look to the opposing symbol to begin the investigation. Sagittarius 23 "Immigrants entering" suggests arriving in a new land, just at the point of stepping onto the shore after a long journey.
It is likely that one have shaky legs, weak knees, or have difficulty finding one's legs after, say, a long journey at sea. But again, this does not cast 23 Gemini/Sagittarius individuals as being 'weak kneed'. It just emphasizes the moment of experience just before stepping onto firm ground.