Crystal Palace goalkeeper Wayne Hennessey did not know what a Nazi salute was when he was charged with making the offensive gesture, says a Football Association panel.
The charge was found not proven this month and Wales international Hennessey, 32, will face no punishment.
The regulatory commission has published its written reasons for the decision.
It said Hennessey showed a "lamentable degree of ignorance" about Adolf Hitler, fascism and the Nazi regime.
Hennessey was pictured with his right arm in the air and left hand above his mouth in a photo posted on Instagram by German team-mate Max Meyer after Palace's FA Cup win over Grimsby on 5 January.
Hennessey denied the charge and said any resemblance to the Nazi gesture was "absolutely coincidental".
The charge was found not proven after two members of the three-man panel believed the photograph had been "misinterpreted" and the other said the "only plausible explanation" was that Hennessey made the salute.
Hennessey said he "waved and shouted at the person taking the picture to get on with it" and "put my hand over my mouth to make the sound carry".
He submitted photographs to the panel of him making similar gestures during matches to attract the attention of team-mates.
The panel said Hennessey was "able to corroborate" his explanation with a series of photographs, including one that showed his right arm raised and left hand across his mouth in a "similar way" to the photo posted on Instagram.
Hennessey said "from the outset" of the hearing that he did not know what a Nazi salute was.
"Improbable as that may seem to those of us of an older generation, we do not reject that assertion as untrue," said the panel.
"In fact, when cross-examined about this Mr Hennessey displayed a very considerable - one might even say lamentable - degree of ignorance about anything to do with Hitler, Fascism and the Nazi regime.
"Regrettable though it may be that anyone should be unaware of so important a part of our own and world history, we do not feel we should therefore find he was not telling the truth about this.
"All we would say (at the risk of sounding patronising) is that Mr Hennessey would be well advised to familiarise himself with events which continue to have great significance to those who live in a free country."
The panel said other photographs from the evening showed Hennessey's arm "raised in slightly different but comparable postures" that "at its lowest" demonstrates he was trying to attract the attention of the photographer, Jordan Bussolini.
It said the FA was "entirely justified" in bringing the case but that "rather than giving a Nazi salute, we think it more likely that Mr Hennessey was, as he says, trying to shout at and to catch the attention of the waiter."