Refers to an inclined surface or pathway, often for vehicles or boats. 'Slip' here means a slipway for launching ships, not a mistake.
港には船を上げるための斜路がある。
There is a slipway at the port for hauling up boats.
駐車場の斜路をゆっくり下りてください。
Please go down the parking lot ramp slowly.
Compound of 斜 (しゃ, 'diagonal, slanting') and 路 (ろ, 'path, road'). The term is straightforward and modern, with no obscure historical derivation.